Blood supplied to a patient must usually be purified by filtration to avoid jeopardizing the patient. The blood may be obtained from the patient during surgery when it is advantageous to store excess blood outside the body to facilitate the surgical procedure, or in blood conservation by scavenging the blood from the wound. Such blood is usually collected in a cardiotomy reservoir and purified there by passage through a filter unit within the reservoir. In cardiopulmonary bypass procedures where an extracorporeal blood circuit is generated and an oxygenator in the circuit takes over the function of the lungs, a cardiotomy reservoir commonly supplies purified blood to the oxygenator. The blood passing through the reservoir must not only be purified of undesirable particulate matter such as surgical debris, but must also be freed of entrained air bubbles before being returned to the patient or supplied to an oxygenator.
It is known to provide in a cardiotomy reservoir a filter unit comprising means for screening out particulate matter, and means for defoaming the blood to remove the air trapped therein. Examples of such known devices include those disclosed in U.S. Pat. Nos. 3,507,395 and 3,768,653. U.S. Pat. No. 3,507,395 to Bentley discloses a cardiotomy reservoir comprising a chamber containing a plate surrounded by a fibrous filter element contained in a nylon bag. The plate first spreads the incoming blood to remove air bubbles therefrom which are vented, or drawn by vacuum, from the chamber, and the filter removes solid particles from the blood as it passes therethrough before leaving the chamber. U.S. Pat. No. 3,768,653 to Brumfield discloses a cardiotomy reservoir comprising a tubular chamber having a tangential inlet for the blood which is directed onto a filter across one end of the chamber which also contains a conical air filter.
Many other filtration systems are known for filtering blood and many use multiple layer elements to remove unwanted materials from the blood as it passes through the layers. U.S. Pat. No. 3,765,536 and U.S. Pat. No. 3,765,537 to Rosenberg disclose a multiple layer blood filter elements including one comprising a first layer of coarse polypropylene netting, a second downstream layer of open-mesh polyester, a third spacer layer of polypropylene netting, a fourth microporous layer and a fifth polypropylene netting spacer layer.
Other commercially available reservoirs contain various arrangements of defoamer and filter layers. However, the known arrangements suffer from the disadvantage of having a limited useful life before one or more of the layers, particularly the filter layers becomes blocked or obstructed. We have now discovered an arrangement whereby the useful lifetime of the cardiotomy reservoir may be extended beyond that hitherto possible without significant blockage or obstruction in normal use and which more efficiently separates entrained gas and particulate matter from the blood.